Xbox360 Backwards Compatibility

You can view the published list of all the games that will be supported at launch Here, but eventually all Original Xbox games will be supported. As long as you have a hard drive that is because the original Xbox has a built in hard drive and Microsoft can't make the original games aware of the presence of a hard drive or choose not to use it.

A couple of nice benefits is that all the Xbox games running on the 360 will be running in 720P, have antialiasing and if they support multiplayer allow you to play multiplayer with people playing the same game on the original Xbox. All downloadable content will be available for download from the Live marketplace and I think Microsoft has done a cracking job.